Field of the Invention
The present invention relates to an ultrasound probe and an ultrasound image diagnostic apparatus.
Description of Related Art
According to ultrasound diagnostics, a realtime display of heartbeat and/or motion of a fetus can be executed by a simple operation of applying an ultrasound probe to a body surface. Additionally, the ultrasound diagnostics are excellent in safety, and enable repeated examinations.
With regard to such technique to display ultrasound images, it has been known that a high contrast image can be obtained by imaging higher harmonic components (e.g. frequencies 2f0, 3f0, etc.) with respect to a fundamental component (frequency f0) of a transmission signal. This imaging method is referred to as Tissue Harmonic Imaging.
The higher harmonic components appear mainly due to nonlinear distortions occurring when ultrasound propagates through a test object. Concretely, when the ultrasound irradiates a living body, the ultrasound signal is distorted due to a nonlinear response of tissue while propagating through the tissue, and thereby the higher harmonic components increase. As a result, a reception signal includes the frequency 2f0 which is twice the frequency f0, and/or the frequency 3f0 which is three times the frequency f0.
As a method for extracting the higher harmonic components in the Tissue Harmonic Imaging, there has been known a filter method and a pulse inversion method.
The filter method extracts, for example, the higher harmonic component 2f0 from the reception signal, by using a bandpass filter having a center frequency of 2f0.
The pulse inversion method transmits the first transmission pulse signal and the second transmission pulse signal obtained by polarity inversion or time reversal with a predetermined time interval, and combines the reception signals so that the fundamental components are canceled and thereby the secondary higher harmonic components are emphasized.
In the meantime, there is a problem that the higher harmonic components contained in the ultrasound signal are easily influenced by attenuation when propagating through a test object because higher harmonic components generally have higher frequencies than that of the fundamental component, and accordingly penetration of the reflected ultrasound signal from a deep part is not good. The penetration can be improved by lowering the frequency f0 of the fundamental component so as to suppress the influence of the attenuation, but a tradeoff relationship occurs in this case, namely, the resolution is degraded.
Among the above two method, the filter method cuts a low frequency region without distinction of fundamental wave and higher harmonics wave, and thereby the influence of the attenuation remarkably appears. In addition, the filter method makes the extracted frequency band narrower, and accordingly the image quality deteriorates more than that of the pulse inversion method. For this reason, the pulse inversion method has become the mainstream in the devices/apparatuses other than low-end devices/apparatuses.
In recent years, there has been proposed a method for improving the image quality of the ultrasound images by using various kinds of higher harmonic components, for example, by using the secondary higher harmonic component and a differential sound component having a lower frequency than that of the secondary higher harmonic component so as to cope with the wider bandwidth ultrasound in the above-described pulse inversion method (for example, see Japanese Patent Application Laid-Open Publication No. 2002-301068).
However, the technique described in Japanese Patent Application Laid-Open Publication No. 2002-301068 has the problem that when the plural kinds of higher harmonic components are used, cancellation among the plural higher harmonics waves occurs due to group delay characteristics of the ultrasound probe. As a result, an intended distance resolution cannot be always obtained.